Anionic polymerization of conjugated dienes with lithium initiators, such as sec-butyllithium, and hydrogenation of residual unsaturation has been described in many references including U.S. Pat. No. Re. 27,145 which teaches a relationship between the amount of 1,2-addition of butadiene and the glass transition temperatures of the hydrogenated butadiene polymers.
The capping of living anionic polymers to form functional end groups is described in U.S. Pat. Nos. 4,417,029, 4,518,753, and 4,753,991. Of particular interest for the present invention are anionic polymers that are capped on one or more ends with hydroxyl, carboxyl, phenol, epoxy, or amine groups.
Anionic polymerization using protected functional initiators having the structure R.sup.1 R.sup.2 R.sup.3 Si--O--A'--Li is described in WO 91/12277 wherein R.sup.1, R.sup.2, and R.sup.3 are preferably alkyl, alkoxy, aryl, or alkaryl groups having from 1 to 10 carbon atoms, and A' is preferably a branched or straight chain bridging group having at least 2 carbon atoms. R.sup.1, R.sup.2, and R.sup.3 are preferably not all CH.sub.3. The bridging group (A') is most preferably a straight chain alkyl having from 3 to 10 carbon atoms and is exemplified by the following compound: ##STR1## which is readily prepared by lithiation of the reaction product of 1-chloro-6-hydroxy-n-hexane and t-butyldimethylchlorosilane. The use of such an initiator as Structure (1) to polymerize the desired monomer(s), followed by capping to produce the second terminal alcohol group, has several advantages over the preparation of telechelic diols by capping polymers prepared with difunctional initiators such as 1,4-dilithiobutane and lithium naphthalide. In addition to providing the option of polymerizing in non-polar solvents, this route avoids the formation of ionic gels, which are known to occur when diinitiated polymers are capped with reagents such as ethylene oxide, generating the polymeric di-alkoxide. These gels form even in relatively polar solvent mixtures and greatly complicate subsequent processing steps. By capping to produce the alkoxide on only one polymer terminus, these gels are avoided.
The initiator of Structure (1) anionically polymerizes unsaturated monomers like conventional lithium initiators but starts the polymer chain with a t-butyldimethylsiloxy functional group that can be converted to a primary alcohol, which is useful in a variety of subsequent reactions. While it appears that the majority of Structure (1) is active if the initiation step is performed at a low temperature (-5.degree. C.), at slightly higher temperatures, a significant fraction of the initiator charge fails to initiate polymerization; a large portion of the initiator is non-reactive or "dead". Initiation with sec-butyllithium occurs efficiently well above room temperature. Nevertheless, the active portion of the initiator of Structure (1) produces living polymers that can be further endcapped and hydrogenated like conventional anionic polymers.
The initiator of Structure (1) affords a polymer chain having a t-butyldimethylsiloxy functional moiety on the end of it. While that protecting group can be removed (deprotection) to give the desired primary alcohol functionality, it is somewhat difficult to practice and is costly. Deprotection of polymers of this type requires contacting with a molar excess (5X stoichiometry) of a strong organic acid, such as methanesulfonic acid, and a compatabilizing cosolvent such as isopropanol (about 20% wt). This mixture is then stirred at elevated temperatures (about 50.degree. C.) until the polymer is deprotected (several hours depending on the specific initiator that is used). When the polymer has been deprotected, it is then necessary to neutralize the acidic hydrolysis catalyst, wash out the spent acid salt, and distill out the compatabilizing cosolvent. These additional steps add time and cost to the process. A functional initiator that contained a protecting group that was easier to remove would be advantaged in processing efficiency.
The polymers derived from initiators of the type described in Structure (1) tend to have a non-uniform microstructure. In the early stages of the polymerization of butadiene using an initiator of this type, 1,4-addition of monomer is the dominant mode of incorporation of butadiene. Even when a solvent system that is high in a microstructure modifier is employed, such as 10% wt diethylether in cyclohexane, the 1,4-addition of butadiene is over 70%. As the polymer grows longer and the C--Li end of the chain distances itself from the t-butyldimethylsiloxy functional end, this effect dissipates and the microstructure of the added units are controlled by the nature of the solvent; at 10% wt diethyl ether in cyclohexane, it would be about 50-60% wt 1,4-addition of butadiene. The adverse effect of this variance in microstructure is manifest in the saturated, hydrogenated, polymer. The segment of the polymer having a linear microstructure, high 1,4-addition of butadiene, becomes a polyethylene segment on hydrogenation and tends to have polyethylene-like crystallinity. This crystallinity tends to increase the viscosity of liquid polymers near room temperature and, in the extreme, may induce the sample to solidify. For the preparation of low viscosity, hydrogenated, functional polymers, an initiator is needed that has protected functionality and allows the preparation of a butadiene polymer that has a uniform microstructure that can be controlled at intermediate levels of 1,4-addition.
It is an object of the present invention to provide improved protected functional initiators that operate efficiently (with a minimum of dead initiator) at economical temperatures. These initiators should operate to afford a butadiene polymer of uniform and controlled microstructure and should be deprotected under mild and low cost conditions.